To satisfy the demands on cars for a smaller size and lighter weight on one hand and a larger living space on the other hand, the reduction of space for the engine room has been required, which leads to the reduction in size and weight of the automotive electrical equipment and the automotive auxiliaries in the engine room, such as alternators, tension pulleys and the like. Also, in response to the demand for quietness, the engine room is closely sealed, so that the temperature in the operating environment will be high. In light of the above, the grease suitable for the rolling bearings, especially for the rolling bearings of the automotive electrical equipment or automotive auxiliaries is required to be resistant to high temperatures and have a long lubrication life.
For the rolling bearings of the automotive electrical equipment or automotive auxiliaries, alkyldiphenyl ethers (ADE) are conventionally used as base oil of the grease. A variety of ADE compounds with excellent heat resistance are proposed (JP Sho 50-73064 A, JP Sho 50-73065 A, JP Sho 55-73791 A, JP 2007-39628 A, WO 2005/040081 A, WO 00/39061 A, and Masatsugu Kohno, “Practical applications and effects of phenyl ether synthetic lubricating oils” Journal of economic maintenance tribology, vol. 417, pp. 18-23, Dec. 5, 2000, Lubrication Technology Inc.
As mentioned above, however, further improvement in heat resistance and lubrication life of the grease composition used for the automotive electrical equipment or automotive auxiliaries is required in line with the recent tendency to reduce the size and weight of those mechanical members and seal them tightly. The conventional ADEs cannot meet those requirements.